Alpha-olefin oligomers and their use as synthetic lubricants ("synlubes") are well-known. Early reports of such synlubes are in Seger et al. U.S. Pat. No. 2,500,161 and Garwood U.S. Pat. No. 2,500,163. U.S. Pat. No. 2,766,312 describes the oligomerization of .alpha.-olefins in a Group IV metal oxide bed using a BF.sub.3 promoter catalyst. Promoters include water, alcohol and ether.
U.S. Pat. No. 3,149,178 describes the preparation of a synlube by oligomerizing a C.sub.6-12 .alpha.-olefin thermally or using a Friedel Crafts or peroxide catalyst followed by distillation to remove dimer. The distillation residue is hydrogenated for use as a synlube. U.S. Pat. No. 3,382,291 discloses a BF.sub.3 -promoter (e.g. alcohol) process for making .alpha.-olefin oligomers in which the BF.sub.3 is used to saturate the .alpha.-olefin feed and a second stream of BF.sub.3 -promoter is fed to the reaction.
Cupples U.S. Pat. Nos. 4,045,507 and 4,045,508 describe a continuous process of oligomerizing .alpha.-olefins using a BF.sub.3 -alcohol system.
Use of hydrogenated oligomers as synlubes depends to a large extent on the viscosity of the hydrogenated oligomer. A large market exists for what is called a 4 cs (centistoke) synlube. These have a viscosity close to 4 cs at 100.degree. C. (e.g. 3.6-4.2 cs). Attempts have been made to make 4 cs synlubes from .alpha.-decene oligomer by topping the crude unsaturated oligomer to remove monomer and dimer followed by hydrogenation of the residue which consists mainly of trimer, tetramer and higher oligomers. These fluids generally have viscosities that are too high to qualify as 4 cs synlubes. Attempts to distill this saturated oligomer to obtain a mainly trimer fraction having the proper viscosity, flash point, volatility and bromine number have encountered problems apparently due to thermal cracking of the hydrogenated oligomer during distillation. Accordingly a need exists for a process that can consistently make a saturated, mainly trimer .alpha.-olefin oligomer having the required viscosity, volatility, flash point and bromine number required for a successful synlube.